The development of new active therapeutic agents is of increasing importance as more bacterial strains resistant to existing conventional antibiotics are emerging. Natural antimicrobial peptides represent one successful form of chemical defense that eukaryotic cells use against bacteria, protozoa, fungi, and virus. Antimicrobial peptides kill bacteria by disrupting their membranes, and these peptides may be developed into a new line of defense against infectious diseases. The main goal of the proposed research is to understand the mechanism and selectivity of a human antimicrobial peptide, LL37, and its derivatives. A combination of structural, dynamics, and thermodynamic studies will be used to investigate the mechanism of membrane-disruption, and the important lipid-peptide and peptide-peptide interactions that determine the specificity for disruption of bacterial rather than eukaryotic membranes. The main analytical tool in these studies is solid-state NMR spectroscopy, using a set of techniques which are well-suited to atomic-level structural studies in non-crystalline membrane systems. We will obtain the following high-resolution structural information about the membrane-disrupting mechanism of antimicrobial peptides: (i) secondary structure; (ii) orientation relative to the membrane bilayer normal; and (iii) dynamics and mechanism of membrane-disruption. The data from these three types of measurements will be combined to obtain a detailed picture of the membrane-bound antimicrobial peptides. The experiments will employ a variety of solid-state NMR methods in order to measure chemical shift and dipolar coupling parameters. Antimicrobial peptides will also be characterized in solution (prior to membrane insertion) and in micelles using circular dichroism and solution NMR experiments. Differential scanning calorimetry, deuterium NMR, and phosphorous-31 NMR experiments will also be used to probe local as well as global changes in lipid motional dynamics upon interaction with the antimicrobial peptides. This proposal aims to further our fundamental understanding of the antimicrobial peptide function to develop a new class of peptides that are more potent and selective than LL37 or related peptides. These peptides have therapeutic potential as antibiotics and have particular importance for cystic fibrosis patients.